This invention relates generally to electronic signal processing circuits and more particularly to radio frequency (RF) mixers.
In the area of radio frequency (RF) receivers or transmitters, mixing circuits (mixers) generally perform frequency translation by multiplying two input signals, each of which may comprise a non-inverted/inverted signal pair. Typically, one input signal is a received signal containing information to be processed, and the other signal is typically a reference signal generated by the mixer or the receiver/transmitter circuits. Mixers have many varied applications. For example, mixers are commonly embodied in equipment used for sending and receiving AM/FM radio signals, both cable and broadcast television signals, and even in cable modems. Mixers may be used in systems designed to send or receive any type of information in an RF signal, such as voice, video, and data. The quality and performance of the mixer generally depends on achieving a closely linear operation and obtaining a controllable and predictable gain. In the prior art, the most commonly used active device mixer is known as the Gilbert cell. Other passive device mixers exist which use arrays of transformers and diodes. However, these passive device mixers are generally not amenable to integrated circuit design. Because its configuration is generally suitable for integrated circuit fabrication, the Gilbert cell has essentially become the standard xe2x80x9con-chipxe2x80x9d mixer configuration. FIG. 1 details the circuit layout of a typical prior art Gilbert cell mixer.
Mixers perform frequency translation by multiplying two frequencies together. The graphs of FIG. 2 give a hypothetical illustration of how two signals are multiplied. We begin with the time domain representation of the two input signals fRF(t) 200 and fLO(t) 201. The local oscillator (LO) signal is, ideally, a square wave, and the RF signal is a hypothetical modulated sinusoidal waveform. Multiplication of these two signals is simplified by transforming each one into the frequency domain and convolving the two transformed signals. Convolution is well known in the art as the method of obtaining the frequency domain representation of two signals multiplied in the time domain. It is also well known in the art to transform a time domain signal into its frequency spectrum through a Fourier Transformation. FRF(xcfx89) 202 and FLO(xcfx89) 203 represent the RF and LO signals in the frequency spectrum. Convolving FRF(xcfx89) 202 and FLO(xcfx89) 203 produces the intermediate frequency (IF) output frequency spectrum FIF(xcfx89) 204. As shown by FIG. 2, the output FIF(xcfx89) 204 contains a scaled version of the RF spectrum centered at the sum of the RF harmonic frequency and the LO first harmonic frequency and another scaled version at the difference of the RF and LO harmonic frequencies. Thus, the product of two signals produces xe2x80x9csum-and-differencexe2x80x9d results. If the mixer is an up-converter, the difference result is filtered out or suppressed while the sum result is processed further. Conversely, if the mixer is a down-converter, the sum result is filtered out or suppressed while the difference result is processed further.
To perform this frequency translation, the Gilbert cell, as shown in FIG. 1, typically includes a mixer core 10, an RF input section 11, and a biasing circuit (not shown). The mixer core 10 is made up of an LO switching interface 12 with input terminals LOP 100, which receives the non-inverted LO input signal, and LON 101, which receives the inverted LO input signal, and an IF output with terminals IFP 102 and IFN 103 for providing the mixer output. The LO switching interface 12 also contains two pairs of transistors, Q3104 and Q6107, and Q4105 and Q5106, wherein the Q3104 and Q6107 pair are each connected to input terminal LOP 100, and the Q4105 and Q5106 pair are each connected to input terminal LON 101. The LO switching interface 12 operates in such a way so as to quickly switch between turning Q3104 and Q6107 on while Q4105 and Q5106 are turned off, and vice versa. Therefore, at any one time, one of the transistor pairs is on and the other off.
The LO input signal which drives the switching process is ideally represented as a square wave to minimize the switching time between the two pairs of transistors. This process is normally accomplished by amplifying an LO sinusoidal signal into compression. By amplifying the signal into compression, the tops of the sinusoid are flattened out or clipped, thus approximating the attributes of a square wave.
The RF input section 11 contains input terminals RFP 108, which receives the non-inverted RF input signal, and RFN 109, which receives the inverted RF input signal; two transistors Q1110 and Q2111, which are arranged as an emitter coupled differential pair; a series connection from Q1110 to the Q3104 and Q4105 of the mixer core 10; another series connection from Q2111 to Q5106 and Q6107 of the mixer core 10; and a pull-down current source 112 connected to the differential pair Q1110 and Q2111. Input terminals RFP 108 and RFN 109 may also be used for receiving a biasing signal from the biasing circuit. With connections to the load resistors RLN 113 and RLP 114 through the mixer core 10 and the resistors RE1115 and RE2116, the differential pair Q1110 and Q2111 operates as a differential amplifier. Further, the active mixer core transistors (Q3104 and Q6107 or Q4105 and Q5106) create a cascode stage in the differential amplifier.
A cascode stage is generally well known in the art as an amplifier comprising a common emitter stage followed by a common base stage. For the cascode connection, either transistor of the differential pair may be considered a common emitter stage. As shown in FIG. 1, the RF transistors Q1110 and Q2111 each form a common emitter stage of the circuit. Which ever of the mixer core transistor pairs is activated at any one time, for example Q3104 and Q6107, will typically form the common base stage. Thus, the common emitter-common base connection produces a cascode configuration. In a cascode configuration, the effective load resistance, as seen by the RF transistors, i.e., the common emitter stage, is not the value of load resistors, here RLN 113 and RLP 114, but rather, is typically the much lower input resistance of the transistors Q3104 and Q6107.
The addition of the cascode stage in the Gilbert cell generally helps avoid frequency response attenuation caused by Miller effect in the common emitter stage. Miller effect arises in the general situation where there is an impedance straddling the input and output terminals of an active network (assuming the impedance does not affect the voltage gain of the network). Miller""s Theorem shows that this straddling impedance is equivalent to the sum of two xe2x80x9cMillerxe2x80x9d impedances which do not straddle the input and output terminals (i.e., one in the input circuit and one in the output circuit). The values of these input and output xe2x80x9cMillerxe2x80x9d impedances depend, in general, on both the value of the original straddling impedance and the network""s voltage gain. In effect, a small impedance straddling the input/output terminals may be equivalent to an input impedance many times the original straddling impedance value. In the situation of a capacitance, the influence of the straddling capacitance, based on Miller effect, may have an enormous effect on the transistor""s high frequency behavior.
A transistor typically has an inherent capacitance between the base and collector, which, in a common emitter configuration, results in effectively straddling the input/output terminals. Because the value of this inherent capacitance will generally be fixed depending on the particular transistor, the voltage gain of the common emitter stage will usually provide a greater influence on the value of the equivalent xe2x80x9cMillerxe2x80x9d input impedance. Because of its configuration, voltage gain across a common emitter will generally depend on the value of the load impedance/resistance. With a cascode configuration, the low load resistance, as seen from the common emitter stage, typically reduces the voltage gain which would then reduce the Miller effect. Therefore, the Gilbert cell""s cascode configuration helps reduce the frequency response attenuation which typically affects common emitter circuits.
The Gilbert cell also includes a biasing circuit with a bias output connected to the RF biasing input terminals for providing the appropriate biasing signal to the RF input section.
In operation, the Gilbert cell""s LO switching interface 12 alternates quickly switching its transistors on and off, which, in turn, provides alternating paths to the IF outputs for the RF signals at RFP 108 and RFN 109. As shown in FIG. 3, the IFP output signal 300, therefore, alternates between the non-inverted RFP signal 302 and inverted RFN signal 303, while the IFN output signal 301, conversely, alternates between the inverted and non-inverted RF signals. The resulting output signal waveform is the sum-and-difference product of the LO and RF input signals. Appropriate selection or manipulation of the LO input signals LOP 304 and LON 305 will typically produce the desired IF output signal.
Ideally, the IFP and IFN output signals 300 and 301 should be exact mirrored images of each other. This mirrored attribute results from the pure sum-and-difference product of the signals, the symmetry and balance in the mixer circuit, and the balance of both the LO and RF input signals. Without symmetry or balance, distortion increases in the output signal due to portions of the original LO and/or RF frequencies translating through to the IF output signal, a phenomenon known as xe2x80x9cfeed-throughxe2x80x9d. LO and RF signal balance means that there is little or no DC component of the differential signal. In other words, the LO signals at the LO inputs must be centered around the same DC level. If not, then, in the mixing process, in addition to the pure LO/RF sum-and-difference product, there is a DC component of the LO signal multiplied by the RF signal, which results in the RF feed-through added to the IF output signal. The same applies for LO feed-through where the RF signal is not balanced. Thus, to prevent LO/RF feed-through, both LO and RF signals must be balanced. This technique for suppressing LO/RF feed-through is generally known in the art as double balancing, and mixers which use it are generically referred to as double-balanced mixers.
In addition to the double balancing attribute, the circuit""s devices should also match as closely as possible. Because the Gilbert cell is designed as a differential amplifier, the transistors of the differential pair, Q1101-Q2111, from FIG. 1, should ideally have identical device specifications. Generally, the further from identical the device specifications get, the more distortion results in the IF output. In the current state of the art, it is possible to achieve very nearly identical device specifications which helps minimize distortion in the IF output.
The Gilbert cell configuration generally provides a closely linear operation even though the transistors which make up the differential and cascode stages are nonlinear devices. The differential setup generates a compensating effect on the nonlinear response of transistors, Q1110 and Q2111. As Q1110 shuts off, Q2111 turns on, thereby passing each other""s nonlinear response paths going in opposite directions. The net effect of this property creates a joint response which is more nearly linear than each transistor""s individual response.
The Gilbert cell mixer also typically produces controllable and predictable gain. Because it is configured as a differential common emitter amplifier, gain is generally determined by the ratio of the load resistors, RLP 114 or RLN 113, to the emitter resistors, RE1115 or RE2116. Therefore, virtually any level of gain may be obtained by appropriate selection of the resistance values.
With all of the advantages of the Gilbert cell configuration, it still possesses limitations in its application. One problem is that the supply voltage to the cell must be divided across four circuit levels. The load resistors, LO transistors, RF transistors, and pull-down current source are connected in series. The constant DC current flowing through the load resistors causes a corresponding voltage drop. The LO transistors Q3104 through Q6107 and pull-down current source 112 also require a certain voltage level to operate properly. Thus, the processing RF transistors Q1110 and Q2111 must be biased from the remaining voltage. For example, if the voltage drop across the load resistor and pull-down current source is 0.5 volts each and each transistor requires two volts for proper operation, five volts is required just to drive the circuit. Considering that the standard supply voltage is five volts, the available headroom is severely limited.
Headroom, as that term is generally known in the art, typically refers to the available peak-to-peak signal amplitude that an amplifier can produce without clipping the top or bottom of the signal peaks. In a common emitter amplifier, the DC operating voltage (Vce at the operating or xe2x80x9cQxe2x80x9d point) is limited at one end by the voltage at which the transistor enters its saturation region and at the other end by the supply voltage, Vcc. The output signal typically alternates about the DC operating voltage. Because the amplifier cannot produce a response which exceeds its limits, the peaks of an output signal cannot exceed either the supply voltage or the saturation voltage without being clipped. Therefore, the closer the operating voltage is to either limit, the smaller the unclipped peak-to-peak signal output can be, which results in limited headroom.
Another problem with the prior art mixer is the distortion caused by the cumulative nonlinear effects of the differential and cascode stage transistors. While distortion due to the differential pair is small because of its natural compensating effect, the signal must still travel through non-differential cascode stage transistors. Thus, the small distortion from the differential pair is substantially increased by the cascode stage transistors. When processing higher frequency ranges or needing a more precise or accurate solution, the distortion attributable to the Gilbert cell arrangement becomes significant.
It would therefore be desirable to have a mixer circuit with increased headroom and improved linearity over the prior art system.
These features are achieved by a system and method which combines the mixer core transistors and the RF input circuit transistors on the same circuit level, i.e., in anti-series. Compared to a series connection, in which devices are connected together at one node and share the same current and a pro ratable portion of the voltage across the connected devices; or a parallel connection, in which the devices are directly connected at two nodes and share the same voltage across each connected device; an anti-series connection connects devices in such a way that they are connected at one node, but do not necessarily share a pro ratable portion of a voltage drop. Instead, the devices are on the same circuit level, but without necessarily being parallel. Thus, the voltage drop across each device is not necessarily equal, as in a parallel connection, but also does not necessarily add up to the total voltage drop across the level, as in a series connection. Removing the series connection across the total available supply voltage increases headroom by effectively raising the available operating voltage for both the RF and LO transistors and increases the linearity of the mixer""s response by eliminating the non-differential cascode stage.
To realize these desirable characteristics, the present invention includes a first and second pair of LO transistors connected respectively to a first and second LO input terminal. It also includes a first RF transistor which is connected to a first RF input terminal and connected in anti-series to the first one of the first pair and to the first one of the second pair of LO transistors. It has a second RF transistor which is connected to a second RF input terminal and connected in anti-series to the second one of the first pair and to the second one of the second pair of LO transistors. It also has two IF outputs. One of the IF outputs is connected to the outputs of both the first one of the first LO transistor pair and the second one of the second LO transistor pair. The other IF output is connected to the outputs of both of the remaining LO transistors.
The present invention produces the desirable characteristics through a method for mixing signals, wherein the circuit receives an RF signal to be mixed. An LO signal is used to continuously alternate switching between enabling the first pair of LO transistors while the second pair is disabled and enabling the second pair of LO transistors while the first pair is disabled. Depending on which of the LO transistor pairs is enabled, the LO and RF transistors form two differential pair amplifiers. The present invention outputs the signal produced by the two differential pairs as the IF signal.
It is an advantage of a preferred embodiment of the present invention to provide a mixer with increased headroom for any given supply voltage. Because the RF and LO transistors operate in anti-series, as opposed to the prior art series configuration, the available operating voltage of the differential amplifiers is now generally increased. This increase in available operating voltage actually increases the headroom for signal processing.
It is another advantage of a preferred embodiment of the present invention to improve the linearity of the mixer""s response. The present invention mixes the signals through two differential pairs on the same circuit level. The prior art system directs the signals through the differential RF stage transistors and then through the non-differential cascode stage transistors before outputting the final IF signal. The present invention increases the linearity of the mixer response by eliminating any non-differential transistor processing.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.